What size 20's are you planning? I have ACS springs, 20" 312's and space saver tire - the spare is about 26.5" overall the same as my 275/30/20 rear tires. Yes you will sacrifice some trunk space. Get a good tight fitting cover and rachet tie down down strap and you're good.

I was looking at the tire size on the Bimmerzone website pic, the picture shows the space saver spare sitting on top of a stock tire and it looks like a taller tire - that's what prompted my query.